1. Field of the Invention
This invention relates to semiconductor devices and to charge-coupled devices. More particularly, the invention relates to a method and apparatus for the transfer of charge accumulated in photoelements of charge-coupled imaging devices into a register in multiple packets.
2. Description of the Prior Art
Charge-coupled imaging devices are well known and widely used. In such devices a linear series of light sensitive imaging elements, or an area array of such elements, accumulate charge in response to ambient conditions. In the case of area imaging devices, by employing a lens to focus ambient light, charge accumulates in individual photoelements as a function of the intensity of the radiation from different portions of a scene focused onto the charge-coupled device. Such area imaging devices may be employed in solid-state video cameras in place of conventional, bulky, heavy and power-consuming tubes.
CCD area imaging sensors typically are fabricated by providing an array of photoelements divided into rows and columns. The photoelements typically comprise transparent electrodes for creating potential wells in the substrate. Disposed between each column of photoelements is a "vertical" shift register extending from the top of the array to the bottom. The column of photoelements is separated from the appropriate shift register by electrodes which create a potential barrier in the substrate. During charge integration period the barrier is maintained at a high level by suitable signals. This allows electrons to accumulate in potential wells established beneath the photoelements. At the end of the integration period, an appropriate signal is applied to the transfer gates to lower the barrier to a potential which allows the accumulated charge to flow into the shift register. Once in the shift register, the accumulated charge may be clocked to the top of the array into a "horizontal" register from which it is shifted to a sensing electrode or other detection apparatus which measures the accumulated charge and provide an output signal.
The sensitivity of a CCD area imager depends directly on the fraction of the area of a unit cell which is responsive. Accordingly, to maximize sensitivity, it is desirable to minimize the area occupied by the vertical CCD register. Unfortunately, the amount of charge which may be transferred from a given photoelement and shifted to the horizontal CCD register is determined, in significant part, by the width of the shift register. Wider shift registers are capable of handling greater amounts of charge than narrow ones.
The problem of maintaining the CCD shift registers at a suitably small size, while handling large amounts of charge, is even more acute in the case of infrared image sensors. Because of the low contrast signal in such sensors, very large levels of signal charge, on the order of millions of electrons, need to be accumulated in individual photoelements.
Two approaches have been proposed for handling such large quantities of charge without increasing the size of the vertical shift register. In "A 480.times.400 Element Image Sensor with a Charge Sweep Device" by M. Kimata et al., 1985 IEEE International Solid-State Circuits Conference, Digest of Technical Papers, pp. 100-101, a technique is proposed by which the charge from a single photoelement in each column of photoelements is shifted into the shift register and allowed to overflow along the shift register in both directions. Once all of the accumulated charge from the photoelement is present in the shift register, the entire vertical register is emptied by being clocked out into the horizontal register. While this approach does improve the charge-handling capability of the image sensor, the sensor operates undesirably slowly because as many cycles of the shift register are required for each photoelement as there are elements in a column. Thus, in the described apparatus with 400 rows of photoelements, the entire 400-bit shift register must be emptied for each photoelement, before the charge accumulated in the next photoelement is allowed to flow into the shift register.
A second approach for increasing charge-handling capability without increasing the size of the shift register is described in "Image Sensor with Resistive Electrodes" by H. Heyns et al., Philips Tech. Review, 37(11/12):303-311. In this proposal, a single long resistive electrode is provided in place of the discrete elements of the shift register. The resistivity of the electrode allows establishing a voltage gradient along the electrode. In operation, all of the charge from a single photoelement is transferred to beneath the electrode where it flows "down hill" to the horizontal register disposed at an end of the column of photoelements. Unfortunately, such an approach is slow at transferring charge unless a steep voltage gradient is established. Steep voltage gradients are difficult to implement in large devices having numerous photoelements. Additionally, the resistivity of the electrode consumes extra power and alters the thermal characteristics of the charge-coupled device undesirably.